Re: [PATCH 0/6 v2] Calculate pcp->high based on zone sizes and active CPUs

[David Hildenbrand]

On 28.05.21 12:09, Mel Gorman wrote:
> On Fri, May 28, 2021 at 11:52:53AM +0200, David Hildenbrand wrote:
> > > > "Disable pcplists so that page isolation cannot race with freeing
> > > >    in a way that pages from isolated pageblock are left on pcplists."
> > > >
> > > > Guess we'd then want to move the draining before start_isolate_page_range()
> > > > in alloc_contig_range().
> > >
> > > Or instead of draining, validate the PFN range in alloc_contig_range
> > > is within the same zone and if so, call zone_pcp_disable() before
> > > start_isolate_page_range and enable after __alloc_contig_migrate_range.
> >
> > We require the caller to only pass a range within a single zone, so that
> > should be fine.
> >
> > The only ugly thing about zone_pcp_disable() is
> > mutex_lock(&pcp_batch_high_lock) which would serialize all
> > alloc_contig_range() and even with offline_pages().
>
> True so it would have to be accessed if that is bad or not. If racing
> against offline_pages, memory is potentially being offlined in the
> target zone which may cause allocation failure. If racing with other
> alloc_contig_range calls, the two callers are potentially racing to
> isolate and allocate the same range. The arguement could be made that
> alloc_contig_ranges should be serialised within one zone to improve the
> allocation success rate at the potential cost of allocation latency.

We have 3 main users of alloc_contig_range():

1. CMA

CMA synchronizes allocation within a CMA area via the allocation bitmap. 
So parallel CMA is perfectly possible and avoids races by design.

2. alloc_contig_pages() / Gigantic pages

Gigantic page allocation could race with virtio-mem. CMA does not apply. 
Possible but unlikely to matter in practice. virtio-mem will retry later 
again.

3. virito-mem

A virtio-mem device only operates on its assigned memory region, so we 
cannot have alloc_contig_range() from different devices racing, even 
when within a single zone. It could only race with gigantic pages as CMA 
does not apply.


So serializing would mostly harm parallel CMA (possible and likely) and 
parallel virtio-mem operation (e.g., unplug memory of two virtio-mem 
devices -- unlikely but possible).


Memory offlining racing with CMA is not an issue (impossible). 
virtio-mem synchronizes with memory offlining via memory notifiers, 
there is only a tiny little race window that usually doesn't matter as 
virtio-mem is expected to usually triggers offlining itself, and not 
user space rancomly. Memory offlining can race with dynamic gigantic 
page allocation, wich is highly unreliable already.

I wonder if we could optimize locking in zone_pcp_disable() instead.

--
Thanks,

David / dhildenb